Here is what I did to create a dual mode install disk.
This disk is used to do installs on emt64 and x86 architecture machines.

First I downloaded the 2 iso's and extracted all the files into 2 different directories.
I used the x86 disk as a base for the disk, and renamed (in the root) image.squashfs to image.x86.squashfs.
I then copied the image.squashfs from the emt64 disk to image.amd64.squashfs.
The root now looks like this:

files is a subdirectory I added, that has the script to build the disk, a portage snapshot and stage1 tarballs for both architechures.
Readme.JD is a file I use to track changes to the disk.
Next I went to the isolinux directory and did some more renaming (since I started with a 32 bit disk, these files are .... 32 bit!!)
ren gentoo gentoo.x86
ren gentoo.igz genx86.igz
I then copied files from the 64 bit /isolinux
copied gentoo to gentoo.a64
copied gentoo.igz to gena64.igz
I then edited the boot.msg to:

you can save some cd space by using 64bit kernel with 32bit squashfs loopfile.
(it will work if the kernel has 32bit emulation (the ones from install cds have it enabled)

you can chroot into 64bit environment with no problems from a 32bit system ran with 64bit kernel. (sounds complicated )

this would need some extra work, however, because:

1. the 64bit squashfs image might hold some extra modules for 64bit kernel
2. the kernels would probably have the same version name and would expect their modules to be in the same dir. (when i build 64bit kernels, i add 64 at the end of their name, so that the modules directory would be different)

btw i saw a couple of weeks ago an archive that contained tools you could use to make a multi-livecd media.

i mean, you could take .e.g damn small linux, mepis, knoppix, etc etc. and throw them together onto one cd-r/dvd-r disc. on boot you would be presented with a menu of available distributions on the disc.